Drill Bit sizing (carb jetting)

Excaliber · #1 (**permalink**) 05-04-2007, 11:23 PM

WHAT is the deal with carb jet sizing? An 85 or a 92 is WHAT in terms of a drill bit size?? A numbered drill bit size (1 to XX)? Letter drill bit size (A to Z)? Fractional drill bit size, now theres one I understand!

I want to drill some jets for experimental use on a small motor. I found a set of 'jet drill bits' for about $250 that are numbered in 'jet size', whatever the heck THAT is! $250 to drill a couple of holes is just to much! Gotta be a cheaper way.

Are some jets actually 'tapered' to increase velocity effect? If so, then drilling a stright hole might not be a good idea.

Anybody got any input on this confusing topic?

1badmofo · #2 (**permalink**) 05-05-2007, 07:16 AM

Look at this.

http://www.jegs.com/webapp/wcs/store...erm=drills+set

Sid

ST · #3 (**permalink**) 05-05-2007, 07:44 AM

Quote:

Originally Posted by Excaliber

Are some jets actually 'tapered' to increase velocity effect? If so, then drilling a stright hole might not be a good idea.

Anybody got any input on this confusing topic?

From Holley:
"Holley jets are broached, flowed, and stamped according to flow rate. NEVER drill jets, as this seriously alters flow characteristics. Stamped numbers are reference numbers and DO NOT indicate drill size."

ENTDOC · #4 (**permalink**) 05-05-2007, 08:12 AM

webers are MM sized, so drilling can be accurate ,it looks as if American carbs are not

Karl Bebout · #5 (**permalink**) 05-05-2007, 08:35 AM

I've always heard that the tapering on the jets makes a difference and that just increasing the size of the hole is not necessarily the way to go. I bought an assortment of Holly jets with at least 30 different pairs for under $50.

Excaliber · #6 (**permalink**) 05-05-2007, 09:49 AM

Here is a pic of a tapered jet from a motorcycle. I'm not sure ALL jets are tapered however. In my case there is no jet available for the carb in the size I want to try, it's either drill it or go with another carb. The 'jet drill kit' I found for $250 was specifically for motorcycle racing purposes.

Rick Parker · #7 (**permalink**) 05-05-2007, 10:21 AM

Flow dynamics is the key here. The tapering (on all jets) is intended to reduce or limit the turbulance created by the restriction. The "hole size" will determine the ultimate flow rate, I think most jets are actually reamed to size. SO........if you have an idea of what size you want, the chapest way out is to find an Industrial Supply house near you and purchase some specific lettered or numbered drill bit(s). Individually they are very inexpensive and are intended to be used in a pin vise. Years ago when I was using DCOE's and when changing cams, pistons, etc, on the cheap we would carefully solder the hole in the jet and carfully drill it with another size bit and then carefully relieve the sharp edge of the hole with a larger bit or very small countersink. We would also run a bit down the interior to remove any excess solder.
This allowed us to get the jetting very close before purchasing the final machined jets, since we didn't have an assortment to draw from. It seemed to work well; that was a long time ago. I would do it again too.

Badger · #8 (**permalink**) 05-05-2007, 01:42 PM

Right on Rick! Flow, not hole size determines how jets are "numbered" . The reason is that a specific drill size does not always give the same flow, as you noted, because of the varying degrees of sharpness at the edges of the drilled hole (the reason for the countersink) can affect the actual flow rate. Carb jets are flowed at the factory and then numbered. Badger

Excaliber · #9 (**permalink**) 05-05-2007, 02:16 PM

Tis better to have a hole that flows enough fuel vs a hole that is to small when you KNOW it's to small. Piece of cake buying the right jet (or several) for a Holley. Try THAT for a vacuum actuated Klien motorcycle carb! The range is severly limited and I'm well outside the 'box' on this project (this thing should rip when I get the carb dialed in).

Maybe I'll try to find some jeweler bit's (do they make those?) and have a go at it, I got lots of jets to play with (all to small).

Excaliber · #10 (**permalink**) 05-08-2007, 05:47 PM

Well I measured the jet hole with a wire guage (like a spark plug gap tool), found the size and then went out to buy the TINY drill bit(s). Tap and Die outlet where the TINY drill bits are in 'number' sizes. Armed with my digital mic I knew what size(s) I wanted in thousands and milimeters as they COULD NOT cross reference a number to a thousands in size!

$30 bucks a pop for each size of bit, I hope I nail the right size jet SOON!

Rick Parker · #11 (**permalink**) 05-08-2007, 06:20 PM

I buy numbered and lettered drills for $1.10 -1.60 each all day long???. What do you get with a $30.00 drill bit in Hawaii............. a Milwaukee variable speed drill, great marketing!

Excaliber · #12 (**permalink**) 05-08-2007, 08:42 PM

It's a .031 size bit (#68), but I think a .032 might be a hair better. They come in a pack of 10, can't buy just one. Perhaps NOW that I know the approximate size I can find a .032 at a better price. But that .031 is doing a fine job for the time being!